The present invention relates to a filled-core optical fiber structure and, more particularly, to a hollow core fiber filled with an optically active material and a method of making such a fiber so that it may be easily coupled to standard optical transmission fiber.
Hollow-core optical fibers have become more prevalent in recent years as various uses for them have been developed. For example, a hollow-core optical fiber may be filled with a liquid crystal material and then used as an electrically-controllable long period fiber grating. See, for example, Electrically Controllable Long-Period Liquid Crystal Fiber Gratings, by Y. Jeong et al., appearing in IEEE Photonics Technology Letters, Vol. 12, No. 5, May 2000, at pp. 519 et seq. Such a fiber has the same essential structure as a common transmission fiber, with the exception of the core region being filled to contain the desired active material. In the arrangement described by Jeong et al., a liquid crystal core fiber was formed by filling a hollow core fiber with a nematic liquid crystal material using capillary action. Index matching between a silica core transmission fiber and the liquid crystal-filled fiber was achieved by using a low index nematic liquid crystal material between the two.
In another utilization of a hollow-core fiber, a two-photon pumped laser has been formed by using a hollow-core fiber that is filled with a particular dye material that causes lasing or superradiance behavior. See, for example, Two-photon-pumped cavity lasing in a dye-solution-filled hollow-fiber systemxe2x80x9d, by G. S. He et al., appearing in Optical Letters, Vol. 20, No. 23, Dec. 1, 1995, at pp. 2393 et seq. In the particular lasing structure as discussed by He et al., the internal diameter of the hollow fiber was 100 xcexcm and the two open ends of the fiber were immersed in two identical liquid coupling cells, each filled with the same dye solution as used in the core of the fiber. Each of the coupling cells further included an optical window to provide coupling out of the liquid-filled fiber and into the rest of the system.
Liquid-core optical fibers have also been used to measure temperature, as disclosed in U.S. Pat. No. 4,201,446 issued to Geddes et al. on May 6, 1980. In the Geddes et al. arrangement, a liquid-core fiber is disclosed in which the refractive index of the liquid core varies with temperature. Therefore, the temperature of the substance through which the liquid-core optical fiber passes can be measured from the maximum angle of the transmitted light output from the end of the fiber. The liquid-core fiber of Geddes et al. comprised a transparent capillary tube that was then filled with the temperature-sensitive liquid. The tube is then joined in series with conventional multimode fibers. There is no discussion in Geddes et al. regarding the degree of optical coupling that could be achieved with this approach, where the use of capillary tubes is known to cause reflections at the interface between the tube and the multimode fiber.
The ability to incorporate optically active materials (i.e., the optical properties of the materials can be altered by various means including the passage of an intense beam of light and the application of an external electric field) into hollow core fibers is of tremendous potential benefit. For instance, materials with high optical nonlinearities can be used to allow for more compact and lower power optical devices. It is asserted that the various prior art approaches to filling and sealing core-filled fibers are not practical for most optical system applications. In particular, the active core material is not sealed inside the fiber, which is a practical necessity for commercial device applications. Further, there is no known low-loss method for coupling light into and out of a core-filled optical fiber.
These and other problems remaining in the prior art are addressed by the present invention, which relates to a filled-core optical fiber structure and, more particularly to method of making such a structure that may be easily coupled to standard optical transmission fiber.
In accordance with the present invention, active core material is first introduced into a section of hollow core fiber, where the fiber is formed to comprise a high index cladding ring that surrounds the hollow core. The active core material may be, but is not limited to, a liquid that functions to alter the optical properties of a signal passing therethrough. Once the core is filled with a desired amount of active material, the end portions of the fiber (which do not contain the active material) are collapsed such that the high index cladding ring is compressed to form a high index core on either side of the core-filled fiber section, essentially xe2x80x9cpinchingxe2x80x9d off and hermetically sealing the active material within the desired section of fiber.
In a preferred embodiment, the collapsing process is performed so that the final fiber structure includes opposing ends where the high index ring material forms high index core regions, followed by transition regions where the high index rings adiabatically taper outward to the central, core-filled fiber section. The adiabatic transition allows for low-loss mode evolution from the high index core region to the high index ring.
It is an aspect of the present invention that the collapsed fiber sections, including the high index core regions, can then easily be coupled (usually conventional techniques such as fusion splicing) to standard transmission fibers, thus forming a low-loss arrangement for coupling into and out of core-filled fibers. Moreover, the collapsed endpoints provide for a hermetic seal and ensure that the core material (in most cases, a liquid) remains in place within the core-filled fiber section.
Other and further aspects and benefits of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.